Headers tube diameter.

MidTexCuda said

I dont think bigger is always the best choice for headers.
Dyi headers can leave you frustrated and cussing the attempt quickl
y. Be warned.
buy a bunch of j bends, collectors, and flanges,from anyone or go to columbia river bends, I think thats the name? Also dont get hung up on dropping alot of coin on merge collectors.

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If the slate is clean, meaning you are building a custom set with the goal to max power in a certain rpm range for a given engine and there are no restrictions because of chassis or usage - I'd agree 100%.
But, once you look at packaging restrictions (suspension, access, etc), limited collector length and size, on an engine that has to make power over a loarge rpm range and various throttle openings, a camshaft that is designed for wide rpm ranges, and feeding into a set of pipes and mufflers downstream... In that circumstance primary and collector size can be boiled down to bigger is better. This is with N/A stuff - not turbocharged.